In principle, mobile magnetic sensing systems can be used by divers or robotic-type autonomous underwater sensing platforms (e.g., Autonomous Unmanned Vehicles (AUVs)) to detect and “home in on” magnetic objects such as underwater and buried mines. This requires magnetic sensing systems that will function effectively onboard highly-mobile sensing platforms such as free-swimming divers or small robotic submarines that are capable of unconstrained three-dimensional motion. Accurate identification and/or neutralization of mine-like objects often requires the sensing platform to approach very close to the object. Therefore, it is desirable that a magnetic sensing system be able to directly and efficiently guide a sensing platform toward magnetic objects or targets, i.e., “to home in on” the magnetic targets. Other potential applications for magnetic sensing systems that involve similar unconstrained three-dimensional sensing platform motion include small robotic flying craft using magnetic sensors to remotely detect and home in on magnetic objects such as land-based mines, camouflaged enemy tanks or even hidden nuclear facilities. In practice, however, the mobile magnetic sensing art has been limited by the fact that the very small magnetic signals of magnetic objects are convolved within the much larger background magnetic field of the Earth.
Magnetically polarized objects or targets such as underwater mines create characteristic DC magnetic field anomalies within the relatively constant background magnetic field of Earth. It is well-known that at distances greater than two or three times an object's linear dimensions, its magnetic signature (measured in “Tesla”) approximates that of a dipole with well-defined mathematical characteristics. However, magnetic dipole field magnitudes decrease with the inverse cube of distance. Thus, at object-to-sensor distances of a few meters, a mine-like object's magnetic signature strength rapidly becomes very small (i.e., on the order of nano-Tesla or 10−9T, or even pico-Tesla or 10−12T) in comparison to the Earth's 50 micro-Tesla (or 10−6T) background field. As a result, a basic challenge for mobile magnetic sensors is the need to discriminate the very small DC target signature components that are convolved with the relatively very large field of Earth. Sensor motion in the very large Earth field can cause relatively huge, orientation-dependent changes in measured vector field components. The large, non-target-related changes in measured field components can overwhelm the relatively small target signatures and thereby reduce the sensor's effective range.
For small autonomous mobile sensing systems, size constraints require that the magnetic sensors be small and operate well with small power and computational budgets. Further, in littoral environments, target localization range can be reduced as the often turbulent, three-dimensional nature of these environments typically will cause large changes in sensor system orientation that will exceed the motion tolerance capability of conventional magnetic sensor approaches. Still further, the operational constraints that are imposed by the naval diving environment largely preclude the practical use of conventional prior art magnetic sensor systems and methods based on magnetic scalar total field or magnetic vector/gradient tensor technologies.
In order to meet the challenges of providing practical and effective magnetic target DLC capabilities for small, highly-mobile maneuverable sensing platforms, U.S. Pat. No. 6,476,610, (i.e., “the '610 patent” as it will be referred to hereinafter) teaches a novel magnetic anomaly gradient sensing system and signal processing concept. The disclosed approach is based on the use of vector triaxial magnetometers (TM) for magnetic field sensing, and the use of triaxial accelerometers for measurement of sensing platform motion and orientation.
Briefly, the '610 patent discloses a target localization approach denoted as Scalar Triangulation and Ranging (STAR). The STAR method uses simplified scalar “contractions” of partial, three-component subsets of the magnetic gradient tensor to determine relative distances to an object, i.e., “triangulate” the object's location. The symmetry properties of the gradient contraction, combined with the '610 patent's sensor array geometries, help to mitigate the adverse effects of large changes in sensor platform orientation. However, the '610 patent does not provide explicit magnetic guidance parameters that can readily be used by an autonomous vehicle to home in on magnetic mines. Further, the '610 patent discloses sensor systems and target localization methods that are too complex and inefficient for direct target-homing guidance. In addition, the partial gradient contraction subsets disclosed in the '610 patent do not provide an optimal parameter basis for homing in on targets of any orientation.
As disclosed in a subsequent related U.S. patent application Ser. No. 10/373,493, filed Feb. 19, 2003, (i.e., “the '493 application” as it will be referred to hereinafter), the symmetry properties of the gradient contraction scalars measured by each single “axis” (i.e., each set of two TMs) can be exploited to provide robotic Underwater Bottom Vehicles (UBVs) with robust two-dimensional magnetic anomaly guidance for homing in on magnetic targets. However, for a certain range of target and sensor orientations, the sensor system embodiments and gradient processing methods that are disclosed in the '493 application may not provide optimal target-homing capabilities. That is, for some sensor-target orientations, the sensor system embodiments of the '493 application may lead a vehicle on a curved path to the target rather than home straight in on the target. Still further, the sensor system embodiments and methods disclosed in the '493 application do not provide robust indications of a mine's vertical position relative to the sensing platform. That is, the '493 application does not disclose a method to explicitly determine whether a target is buried below the level of the searching platform or tethered above the searching platform.
In another related U.S. patent application Ser. No. 10/789,481, filed Mar. 1, 2004, (i.e., “the '481 application” as it will be referred to hereinafter), an advanced multi-sensor array concept is disclosed that can be used for detection, localization and classification (DLC) of mines by autonomous swimming or flying vehicles that are capable of unconstrained three-dimensional motion. However, while the magnetic sensor systems and methods taught by the '481 application have many advantages for DLC of mines using high-mobility sensing platforms, the sensor system embodiments disclosed thereby are more complex than necessary for simple guidance of a vehicle to a magnetic target.